add (+ tab-width) indentation proposals

[s9gf4ult]

This adds indentation proposals on (+ tab-indent)

for example, if you write

newtype SessionId = SessionId

then default indentation proposals would be

newtype SessionId = SessionId
{ getSessionId :: Text

or

newtype SessionId = SessionId
                    { getSessionId :: Text

this patch adds also

newtype SessionId = SessionId
    { getSessionId :: Text

and

newtype SessionId = SessionId
                        { getSessionId :: Text

variants to indent.

In fact, it just duplicates (+ tab-width) for each found indentation proposal.

Unfortunately my code is hackish and this feature might be done better.

[gracjan]

Related #366.

[gracjan]

Thanks for tackling this one. This is important feature.

Having said that I think it should be done a indentation style setting. Either tab-width is important or alignment is important, not both.

Rationale:

Indentation in Haskell is mostly deterministic except in a couple of places when a layout block ends. Keeping it as deterministic as possible will enable us to have a sane indent-region.

Your commit as it is now would increase non-determinism, which is a very sad trade off to take.

[s9gf4ult]

This patch is very stupid and code in haskell-indentation.el is too complicated to understand it in measurable time. I think this patch is the best I can offer.

Or you just mean that I must use haskell-indentation-layout-offset (or some other setting) instead of tab-width ?

[gracjan]

Your patch is an important vote for shallow indentation. Thanks.

There is a place in the code where indentation offset for the { is calculated. Can you find it?

[s9gf4ult]

Well, there is many places where { pattern occures in code. haskell-indentation.el has very very complex code. To do this patch I just grepped possible-indentations defined in haskell-indentation-parse-to-indentations assuming it is a list of posible indentation positions. I found one such place, which is function haskell-indentation-push-indentation, then I modified this function and trick worked.

Code in haskell-indentation.el is not very functional-style, everything mutates everything, it is very complex to understand. We need someone who knows how all this works to make this feature right.

[gracjan]

That is why current project focus is lowering the entry barrier for new users, new issue reporters and first time contributors and the goal is 100 haskell-mode contributors.

We need to leave after us a maintainable piece of software. I've started doing this by writing some unit tests like in #562.

While you tried to hack on haskell-indentation you have learned a lot already. Can you document your knowledge in the form ofo unit tests, comments or refactorings?

[s9gf4ult]

Well, I will try, but can not promise anything usefull.

[gracjan]

Great! Visit us on irc if you need some help!

[gracjan]

I'm closing this PR with strong conviction that #366 is the way to go.